Okay, this is a cheat, but the moment in An Adventure in Space and Time where Matt Smith's Doctor appears at the TARDIS consol in front of William Hartnell (played by David Bradley) is pure magic. It’s such a good trick, it refuses even to be contained by the critical praise of that opening sentence. Was it William Hartnell's Doctor meeting Matt Smith? Perhaps it is Hartnell hallucinating the future of the programme’s character – his character – which happens to
crystallise as Matt’s Smith’s Doctor? Maybe it is David Bradley sizing up his own chances of appearing alongside future Doctors as the First Doctor?
  Wheww! – I feel giddy. The joy lies in the numerous writerly combinations, like one of those childrens' flip books where you can mix faces, jackets and trousers. It's also a great way to breach the cosy realism of the docu-drama as a tribute to the very first episode, when the locked gates of Foreman's yard mysteriously facilitate the audience. Gatiss has created a huge moment that lives up to the huge task of abbreviating the huge experience of Doctor Who, which, like an onion memory, is about peeling away the intertextual layers without ever arriving at the core.